The present invention concerns a method of determining the contour of a depression in the thin-layer region of a solid substrate. The method comprises generating a beam of ions from a starting substance converted to either a gas phase or a vapor phase, deflecting the beam to the surface of the substrate, removing defined layers of the surface by ion-beam sputtering, and determining by means of a probe and a downstream detection-and-processing circuit the concentration of the components removed from the surface.
The determination of depression contours is an analytic task of significance to the overall field of surfacing and coating not only in research and development but also in the control of production and fabrication. Layers as thick as single atoms are removed in the Z direction and the concentration in the flow of removed particles or particular surface layer exposed are measured with special probes as the removal progresses. Examples are the distribution of the element boron (B) in silicon (Si) or of aluminum (Al) in gallium arsenide (GaAs). Depression-contour analysis is of particular importance in micro-electronics, where the distribution of the doping element, in single silicon crystals for example, or the composition of interfaces, in gallium-arsenide layers for example, are measured. High depth resolution and sensitivity are significant requisites.
The aforesaid method combines the continuous breakdown of the surface by ion-beam sputtering, ion-bombardment dusting in other words, with the most sensitive and continuous determination of the particular composition of the surface of the components of interest. The surface of the particular solid substrate being analyzed is removed as uniformly as possible with an appropriate ion beam. The rate of removal can be determined empirically and depends with considerable reproducibility on the homogeneity of the beam's current density, on the type of ion and energy, and on the material itself. Inclusions in the material do not generally much affect the rate of removal if at all.
Uniform removal can be ensured either through the homogeneity of the beam's current density or by scanning the area to be removed with a tightly bundled beam. A primary scanning ion beam erodes the surface. The particular surface composition is then continuously determined inside the resulting crater with a secondary ion mass spectrometer (SIMS). The deep distribution of the individual components can then be determined with the known and common combination of sputtering and SIMS analysis.
State-of-the-art sputtering is carried out with ions of the inert gases, of cesium, or of diatomic oxygen.
Along with actual removal, a surface-sensitive probe will also detect the concentration of interesting components in whatever surface is exposed. The result is a specific concentration c=f(z) of the components in question at every exposed depth z. Depth z can itself be both calculated from the intensity of the beam. From the area density j of the beam, that is, and determined by a concomitant method, specifically by mechanically determining the depth of the crater. The reproducibility and unambiguousness of depth determination are ensured empirically.
Probes appropriate for the present method are also known. The methods employed with such probes can determine the surface concentration of one or more components with sufficient sensitivity. The methods appropriate for such purposes include for example Auger electron spectroscopy, photo-electron spectroscopy, and especially the various types of surface mass spectroscopy. The probe should to the greatest extent possible address only the uppermost layer of atoms. In that event, the depth resolution will be limited only by the uniformity of removal, corresponding to the ion bombardment. The uniformity of solids removal by the particular ion beam employed will accordingly determine the attainable depth resolution f(z) of the overall process if the probe's information depth is assumed as a determinative parameter.
The ionic bombardment not only removes particles from the surface. It also results in intermingling between regions in the vicinity of the surface that the ion beam can enter with a certain probability, a intermingling that is detrimental to the precision of the deep-contour analysis. Assuming that a closed layer of atoms of a foreign element is included to a specific depth z.sub.1 in the solid material, the original and true depth distribution will correspond to a rectangle in the concentration graph c=f(z).
The action (transmission of energy and pulse) of the beam of ions penetrating the surface region will, however, result in such intermingling even before the layer of foreign element has been exposed. This intermingling will diffuse the actual contour detected by the probe. The contour's resolution is accordingly inherently limited, and one object of the present invention is to minimize the intermingling.
The use of ions for the destructive analysis of depression contours is accordingly known. The ions are of the lowest possible energy and the largest possible mass and impact the surface to be removed at the most acute angle possible. If, as is common, the ions are Ar.sup.- ions with an energy E of 1 keV, the angle e of impact will be 70.degree. to ensure the best possible constellation of parameters.
Any additional decrease in bombardment energy E will lead to problems in that their space charges make ion sources very difficult to focus at low energies, limiting the potential for scanning with a narrow beam. Furthermore, since the current densities, and hence the energies available for sputtering per unit of area and time, are all low, it takes much too long to measure a specific depth z.